1. Field of the Invention
This invention relates to a brake disk material for railroad vehicles which is used as a material for brake disks for use in high-speed railroad vehicles.
2. Description of the Prior Art
As a brake disk material adopted for disk brake systems in railroad vehicles having a maximum service velocity of around 100 km/h, there has hitherto been used a pearlitic-matrix flaky graphite cast iron which, according to Japanese National Railway Standard (hereinafter referred to JRS) 12209-1, has a tensile strength of at least 274 MPa and a hardness of 183 to 269 HB. As a brake disk material adopted for high-speed railroad vehicles with a maximum velocity reaching 220 km/h, on the other hand, there has been used a low-alloyed pearlitic-matrix flaky graphite cast iron (Ni: 1.0 to 2.0% by weight, Cr: 0.3 to 0.6% by weight, Mo: 0.3 to 0.5% by weight, hereinafter referred to as "NCM cast iron") which, according to JRS 12209-2, has a tensile strength of at least 245 MPa and a hardness of 210 to 280 HB.
The brake disk material constituted of the NCM cast iron is characterized by an appropriate average coefficient of friction, a stable instantaneous coefficient of friction and a low rate of wear, which ensure a braking force fulfilling the specifications both at the time of service braking for stop (initial speed braking: 35 km/h) and at the time of emergency braking for stop (initial speed braking: 220 km/h). The brake disk material of the NCM cast iron is thus superior to general-purpose pearlitic-matrix flaky graphite cast irons in resistance to heat cracking at the sliding surfaces of brake disks, even under high-frequency (service braking) or high-load (emergency braking) frictional conditions.
The NCM cast iron is obtained through increases in the size and number of graphite particles formed in the structure of the metal to serve for absorption and relaxation of thermal stress, with the attendant reduction in strength being compensated for by reinforcement of the matrix structure through alloying with Ni, Cr and Mo. Comparable to JIS FC25 grade cast irons in strength, the NCM cast iron has good thermal conductivity and excellent resistance to mechanical damage caused by heat loads.
Recently, a maximum velocity of 240 km/h has been realized with railroad vehicles, and a still higher velocity is being sought. Under such existing conditions, the brake disk materials conventionally used have been confronted by new problems. Namely, in case of a high-load braking with the brake disk material in such high-speed vehicles in an emergency, the heat crack resistance of the sliding surfaces, even if formed of the NCM cast iron, may be unsatisfactory.
In view of the above, it has been contemplated to prevent the generation of heat cracks by using a material of further higher strength and toughness, and a forged steel brake disk material has recently been developed and put to practical use. However, the original reason for the use of cast iron in place of steel as a brake disk material is the stable friction-wear characteristics of the cast iron during braking, due to the presence of free carbon (graphite) in the cast iron. Therefore, the forged steel brake disk material has the problems of instability in coefficient of friction during braking and a greater permanent set of the material in use, as compared to the cast iron brake disk material.
With the increasingly higher velocity recently intended for railroad vehicles as mentioned above, the brake disk materials for use in such railroad vehicles are required to have a good balance of friction-wear characteristics, material strength and thermal damage resistance. Furthermore, in order to meet the requirement for smaller non-suspended weight on a design basis, development of a nonferrous brake disk material of high performance has been desired. A typical example of lightweight nonferrous metals is aluminum alloy. When a conventional aluminum alloy is used as a brake disk material for railroad vehicles, however, the material does not show satisfactory friction-wear characteristics or the like in such application, and leads to large disk wear as well as seizure or the like, by which the pad material as a mating material is also consumed.